New Pentair Pump: Priming issue

[Kevin Gobeil]

Installed new Pentair pump but do so by buying individual components. The pump doesn't seem to fully prime, even at full speed it doesn't completely fill the suction strainer up to the cover, the level is somewhere between the top of the strainer basket and cover, and at lower speeds it actually loses prime. Of course I looked for suction side leaks, I do not have a high temp pump union on the suction side, just a male adapter, but I have the gap between the pump inlet flange and the top of the top of the adapter (remaining threads area) sealed with silicone RTV, this is how the original pump was set up, I just had to replace the RTV once in a while when it degraded. After the install I was made aware of special pump unions, but at this point to install one on the suction side I would have to rebuild the suction side and replace 3 3-way valves and potentially 2 vacuum breakers. To investigate as a suction leak, I replaced the stem and body O-rings on all the Pentair 3-way valves on the inlet side, I re-lubed O-ring and over-tightened the strainer cover. I checked the drain plugs for O-rings, lubed them and retightened. I'm starting to wonder about things outside of a suction issue, potentially a problem with the pump and/or incompatibility of parts. As far as configuration, I installed a new Pentair impeller 073129 into a new pump housing 357149, with a new diffuser 072927, with new variable speed motor Century ECM16SQU (1.65 HP, square flange), this seemed to be a compatible configuration. Any thoughts where to look?

 

[1poolman1]

Installed new Pentair pump but do so by buying individual components. The pump doesn't seem to fully prime, even at full speed it doesn't completely fill the suction strainer up to the cover, the level is somewhere between the top of the strainer basket and cover, and at lower speeds it actually loses prime. Of course I looked for suction side leaks, I do not have a high temp pump union on the suction side, just a male adapter, but I have the gap between the pump inlet flange and the top of the top of the adapter (remaining threads area) sealed with silicone RTV, this is how the original pump was set up, I just had to replace the RTV once in a while when it degraded. After the install I was made aware of special pump unions, but at this point to install one on the suction side I would have to rebuild the suction side and replace 3 3-way valves and potentially 2 vacuum breakers. To investigate as a suction leak, I replaced the stem and body O-rings on all the Pentair 3-way valves on the inlet side, I re-lubed O-ring and over-tightened the strainer cover. I checked the drain plugs for O-rings, lubed them and retightened. I'm starting to wonder about things outside of a suction issue, potentially a problem with the pump and/or incompatibility of parts. As far as configuration, I installed a new Pentair impeller 073129 into a new pump housing 357149, with a new diffuser 072927, with new variable speed motor Century ECM16SQU (1.65 HP, square flange), this seemed to be a compatible configuration. Any thoughts where to look?

Your motor's total horsepower rating is 1.65. Your new impeller will load that motor to 2.25 total horsepower. That .6 THP can cause you issues like the motor overheating, though variable-speed motors can be overloaded a bit, just not good practice or recommended.

Your valves in front of the pump should always be suspect. Get new lid and stem O rings, lightly lube them and reinstall. Unless these are very old Compool valves or Ortega valves, it doesn't matter the brand of valve you get the O ring for, they are all the same.
If the valves are old, after replacing the stem O rings (usually 2 per stem) lube them, wrap two layers of Teflon tape around them, lube that, and install. The valve lid can also get worn and not seal.
What are the two "vacuum breakers?"

 

[Kevin Gobeil]

You think I should consider a different impeller? This one appears to be shown as compatible with the motor, but this may be bad information or I may be misunderstanding it. As for the Pentair 3-way valves on the suction side of the pump, I've already replaced all the O-rings. New lubricated o-ring on the cover too. The vacuum breakers are per code and are connected to the pool and spa suction to break suction in the event that someone gets stuck on one of the returns.

 

[1poolman1]

You think I should consider a different impeller? This one appears to be shown as compatible with the motor, but this may be bad information or I may be misunderstanding it. As for the Pentair 3-way valves on the suction side of the pump, I've already replaced all the O-rings. New lubricated o-ring on the cover too. The vacuum breakers are per code and are connected to the pool and spa suction to break suction in the event that someone gets stuck on one of the returns.

As a contractor, I wouldn't put that impeller on that motor as I would be liable and have to do the repair for free if it did cause an issue. Did it once at the very beginning of my business and never made that mistake again. Had to replace a week-old motor that burned up due to the overload.

Are you pulling in air? After the pump starts and goes through its prime cycle, release any air from the filter. Come back a while later and try again. If there is just a tiny bit you likely don't have a problem. Sometimes pumps just don't fill the pot all the way and a variable-speed one seems to do that more often. When they go to lower RPM, it seems they all appear to have air in the pot, mine included (which almost never fills completely with water even on the higher speeds).

I don't believe it is air in the pot at the lower speeds, if all else is sealed and there is no suction leak, but an area of low pressure that can't be filled with the slow-speed water. I have no way to test that other than I don't get air into my filter at the lower speeds.
Follow this blog long enough and you will find dozens of references to that fact.